Roof Sheathing Tolerances

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U.S. Department of Housing and Urban DevelopmentOffice of Policy Development and Research

Roof Sheathing

Connection Tolerances


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Roof Sheathing

Connection Tolerances

Prepared for

U.S. Department of Housing and Urban DevelopmentWashington, DC

by

NAHB Research Center

Upper Marlboro, MD

August 2003


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Table of Contents

Page

Acknowledgements...................................................................................................................... viii

Executive Summary.........................................................................................................................x

1.0 Introduction..........................................................................................................................1

2.0 Testing Plan .........................................................................................................................1

2.1 General.....................................................................................................................12.2 Sheathing Uplift Tests..............................................................................................6

2.3 Diaphragm Shear Tests ............................................................................................8

3.0 Test Results..........................................................................................................................9

3.1 Sheathing Uplift Tests..............................................................................................93.2 Calculated Uplift Capacity.....................................................................................123.3 Diaphragm Shear Tests ..........................................................................................13

4.0 Evaluation of Results .........................................................................................................164.1 Uplift Tests.............................................................................................................16

4.2 Shear Tests .............................................................................................................17

4.3 Edge Distance Effects ............................................................................................18

4.4 Nail Spacing Effects ..............................................................................................194.5 Nail Overdrive Effects ...........................................................................................19

5.0 Conclusion and Recommendations....................................................................................20

6.0 References..........................................................................................................................21

Appendix A Panel Withdrawal Tests - Fastener Numbering

Appendix B Estimated Wind Speeds and Diaphragm Shears

Appendix C Tributary Loading Areas for Nail Withdrawal

Appendix D Metric Conversion Factors


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List of Tables

Page

Table 2.1 Test Matrix...............................................................................................................2

Tabel 3.1 Uplift Test Results .................................................................................................11

Table 3.2 Shear Test Results..................................................................................................16

Table 4.1 Change in Capacity for Tested Nailing Characteristics.........................................16

Table 4.2 Calculated Velocity Pressure and Shear Loads .....................................................18

Table 5.1 Suggested Nailing Tolerances................................................................................21


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List of Figures

Page

Figure 2.1 Baseline Nailing ......................................................................................................... 3

Figure 2.2 Tributary loading areas for 12-inch and 6-inch nail spacingat intermediate supports .......................................................................................................... 3

Figure 2.3 Edge distance variations............................................................................................. 4Figure 2.4 Nail spacing variations ............................................................................................... 4

Figure 2.5 Nail over-drive target variations................................................................................. 5

Figure 2.6 Testing apparatus without sheathing .......................................................................... 7

Figure 2.7 Specimen setup with sheathing attached .................................................................... 7Figure 2.8 Diaphragm specimen in shear racking apparatus ....................................................... 9

Figure 3.1 Uplift failure locations (Regions A, B and C).......................................................... 10Figure 3.2 Specimen near ultimate load .................................................................................... 11

Figure 3.3 Specimen immediately after failure.......................................................................... 11Figure 3.4 Diaphragm specimen under load .............................................................................. 13Figure 3.5 Failure locations (Regions D,E,F, and G) ................................................................ 14

Figure 3.6 Fastener tear-through failure at region D ................................................................. 14

Figure 3.7 Fastener tear-through failure at region E.................................................................. 15Figure 3.8 Fastener tear-through failure at region F .................................................................. 15

Figure 3.9 Fastener tear-through failure at region G ................................................................. 15

Figure 3.10 Rafter splitting and tear-through ............................................................................ 16

Figure 4.1 Ineffective nailing for Edge Distance #3 .............................................................. 19

Figure 4.2 Overdrive depth to capacity loss relationship .......................................................... 20


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Acknowledgements

This report was prepared by the NAHB Research Center, Inc. under the sponsorship of the U.S.

Department of Housing and Urban Development (HUD). We would like to acknowledge the

following individuals for their work on this project:

Jay P. Jones, P.E., Research Engineer

Bryan Adgate, Research EngineerVladimir Kochkin, Research Engineer

Stuart Denniston, Laboratory Support

Lynda Marchman, Administrative Support


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Executive Summary

Proper attachment of structural sheathing to the roof framing is a critical step in building a wind

resistant structure. Inadequate fastening of roof sheathing panels is one of the primary causes of

damage to light-frame wood construction during high-wind events such as hurricanes and

tornados. The current guidelines for placement of sheathing fasteners when used with 2x lumberimply the degree of accuracy that is impractical for typical construction methods. Although it is

reasonable to assume that some amount of variance from these guidelines is allowed, noapproved or validated range of tolerances have been established. The following are standard

specifications for installing wood structural sheathing panels:

minimum 1/8-inch gap between panel edges;

3/8-inch minimum edge distance;

fastener head flush with sheathing surface; and,

specified nail spacing (e.g., 6 inches on center).

This study addresses the need for practical roof sheathing nailing tolerances. These toleranceswill allow for moderate errors in nail placement without compromising the intended strength of

the sheathing connection. This testing plan was developed to establish tolerance limits for:

nail edge distance;

nail spacing ; and,

nail overdrive.

The tests conducted in this research demonstrate that slight nailing discrepancies have littleaffect on sheathing connection capacities. Nailing tolerances, which allow for practical degrees

of variation, were developed from the findings of this study for nail diameters of 0.131 or less

and sheathing thickness of 7/16 or greater and are listed in the following table:


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Minimum

Maximum

CURRENT

GUIDELINES

SUGGESTED TOLERANCES

(NAIL DIAMETERD 0.131 AND SHEATHING THICKNESS T7/16)Edge Distance:

3/8-inch from edge of panel to

centerline of fastenerTolerance:

Minimum = 1/4-inch from edge of

panel to centerline of fastener

Maximum = 3/4-inch from edge of panel to centerline of fastener and

fastener angled such that no portion

is exposed from the underside and

the framing member remains un-split.

Spacing Between Fasteners:Spacing as specified

(e.g., 6 inches on center)Tolerance:

1. Spacing between any twoconsecutive fasteners not to exceed

specified spacing plus one inch.

and

2. Total distance between any three

consecutive fasteners not to exceed

minimum spacing multiplied by two,plus two inches.

and

3. Total number of fasteners in a nail

line shall not be less than calculated

using the minimum required

spacing.

Fastener Overdrive:

Head of fastener is to bedriven flush with sheathing.

Tolerance:

Overdrive allowed if sheathing hasbeen sized thicker than required by

design. Depth of penetration must

be kept such that remaining

thickness under the head of the nail

is greater than or equal to therequired thickness but shall not

exceed 1/3 the thickness of the

panel.


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1.0 Introduction

Proper attachment of roof structural sheathing is a critical step in building a wind resistant

structure. The roof sheathing serves multiple structural functions. It provides diaphragm action to

resist lateral forces, braces framing members, and provides support for the roofing material.

Therefore, the roof sheathing must be properly attached to achieve the intended purpose.Inadequate fastening of roof sheathing panels is one of the primary causes of damage to light-

frame wood construction during high-wind events such as hurricanes and tornados [1][2].

Factors such as fastener edge distance, overdrive, and spacing can affect both the withdrawal and

shear resistance of sheathing fasteners. Yet, current guidelines for attaching sheathing panels to

roof framing do not include acceptable tolerances for these fastening characteristics. Installationguidelines require a minimum 3/8-inch nail edge distance and a minimum 1/8-inch gap between

adjacent panels [3]. When sheathing panels are used with 2-inch nominal width (1.5-inch actual)

framing members in a manner consistent with these minimum placement specifications, the levelof required precisions is such that each fastener should be located within a region less than 5/16

inch. In addition, fasteners are required to be driven flush with the surface of the panel. Becausesheathing panels are typically attached using pneumatic nail guns, a certain degree of deviationfrom the minimum installation specification can be expected. Therefore, a set of acceptance

criteria in the form of maximum allowable tolerances is needed such that both the framer and the

inspector can objectively evaluate sheathing fastener installation.

The objective of this testing program is to investigate the sensitivity of the resistance of roof

sheathing connections to incremental increases in deviation from the baseline installation

practice for nailing characteristics including edge spacing, fastener spacing, and degree of nailover-penetration (i.e., overdrive). Based on this data, acceptable variances can be established that

allow for a reasonable degree of deviation without compromising the structural integrity of the

system. The recommendations developed in this testing program could be used by framers,building officials, and in framing quality programs.

The specific objectives for this test program were to:1. Conduct uplift and shear tests of roof sheathing connections assembled with an

incremental degree of deviation from the baseline scenario;

2. Compare test results with baseline resistance and with published capacities to determine

the effect on strength for each level of deviation; and,3. Use the results of the testing program to make recommendations for installation

tolerances.

2.0 Testing Plan

2.1 GENERAL

Sheathing connection specimens were tested under two loading conditions: uplift and shear.Variations of each nailing characteristic (edge distance, nail spacing, and overdrive) were first

tested in uplift. The variations that exhibited excessive strength loss in the uplift tests were

eliminated as possible tolerance limits and not tested in shear. In addition, the nailingcharacteristics that only varied the spacing between fasteners were not tested in shear since the


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total number of nails in the shear line remained constant. Eight variations in fastener installation

were tested in uplift and three variations were tested in shear. Three tests for each variation weredone for uplift, and two tests for each variation were done for shear. Table 2.1 shows the test

matrix for this plan.

TABLE 2.1

TEST MATRIX

NUMBER OF TESTSNAILING CHARACTERISTIC

Uplift Shear

Baseline 3 2

Edge Distance #1 (3/4-inch, angled 20o) 3 -

Edge Distance #2 (1/4-inch) 21 2

Edge Distance #3 (1/8-inch) 3 -

Fastener Spacing #1 (+ ) 3 -

Fastener Spacing #2 (+ 1) 3 -

Overdrive #1 (Up to 25%) 3 2

Overdrive #2 (25% to 50%) 3 -

Total Number of Tests 23 61The third test for this nailing characteristic was inadvertently not conducted.

The type of fastener used in this study was an 8d common, full round head nail (0.131 x 2-1/2), which follows the IRC 2003 [4] specification for attaching roof sheathing. Although it is

common for builders to use smaller, pneumatic nails (e.g., 0.113 x 2-3/8) for this application,

the larger common sized nail was tested to examine potential negative effects of the largerdiameter on edge distance requirements.

The following sections describe the nailing variations tested in this study.

Baseline

The baseline tests were used as a benchmark for this study. The specimens were constructed in

accordance to panel manufacturer nail placement recommendations (i.e., 3/8 inch from the panel

edge, no overdrive, and zero nail-to-nail spacing tolerance (see Figure 2.1)[3]. The nail spacing

schedule followed the requirements of the IRC 2003 [4]. For one- and two-family dwellings inareas with moderate design wind speeds (i.e., less than 110 mph), 8d common nails at 6 inches

on center at panel edges and 12 inches on center in the field are required for the attachment of

roof sheathing. Near roof edges, ridges, and eaves (within 48 inches) the code specifies 6-inchspacing at panel edges, and 6-inch spacing in the field. The latter nailing schedule was used in

this test program in order to examine the most critical case for the edge nailing. At 12-inch field

spacing, the critical nail (first to fail) is invariably an intermediate field nail. This was confirmedanalytically, and empirically in APAs test report T92-28 [5], which demonstrates how the

tributary loading area for field nails is approximately four times greater than the tributary area

for the edge nailing when 12-inch spacing is used in the field. The 6-inch field nailing criteriaused for this study reduced the tributary loading ratio to approximately two (Figure 2.2) and

therefore could more effectively identify potential capacity losses due to edge nailing

discrepancies (e.g., close edge distances, overdriven nails, or varied nail-to-nail spacing).


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BASELINE

Figure 2.1 Baseline Nailing

Figure 2.2 Tributary loading areas for 12-inch and 6-inch nail spacing at intermediate supports

Edge Distance

Three variations in edge distance were tested: 1/8-inch, 1/4-inch, and 3/4-inch (see Figure 2.3).The 3/4-inch edge distance (Edge #3) required the edge-nail to be angled at approximately 20

degrees to keep the shank fully embedded in the framing member. A nail driven with a portion of

its shank exposed from the underside was considered to be out of tolerance. All nails in this testprogram were kept within tolerance.


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EDGE #1 EDGE #2 EDGE #3

Figure 2.3 Edge distance variations

Nail Spacing

Two variations in nail spacing were tested: one with the spacing increased by 1/2-inch, and the

other with the spacing increased by 1-inch (see Figure 2.4). The number of fasteners (nine perintermediate support) along with the 3/8-inch edge distance requirement remained consistent

with the baseline case. The varied spacing characteristic was applied on only one of the

intermediate-framing members.

SPACING #1 SPACING #2

Figure 2.4 Nail spacing variations


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Nail Overdrive

Two variations were tested for nail overdrive: one up to 25 percent of the panel thickness, and

the other up to 50 percent of the panel thickness (see Figure 2.5). The edge distance and spacing

between fasteners were consistent with the baseline specifications. All fasteners for these tests

were over-driven. However, precise and consistent over-drive was not achievable. Therefore, theactual penetration was measured after each nail had been installed. Tables 2.2 and 2.3 show the

overdrive measurement data.

OVERDRIVE #1 OVERDRIVE #2

Figure 2.5 Nail over-drive target variations

TABLE 2.2

ACTUAL NAIL OVERDRIVE DATA (Penetration #1)(25% overdrive

1)

Test

#

Maximum

Overdrive

(in)

Fastener

Number

at max.2

Minimum

Overdrive

(in)

Fastener

Number

at min.2

AVG

Overdrive

(in)

Std

Dev

(in)

COV

Initial

Failed

Fastener2

Overdrive

at initial

failure

(in)

1 0.123 33 0.056 1 0.092 0.017 18% 15 0.101

2 0.178 15 0.065 1 0.101 0.026 26% 34 0.138

3 0.159 24 0.064 41 0.097 0.022 23% 29 0.1041Target overdrive = 0.25 x 7/16-inch = 0.109 inches2See Appendix A for fastener numbering and locations on test panel.

TABLE 2.3

ACTUAL NAIL OVERDRIVE DATA (Penetration #2)(50% overdrive

1)

Test

#

Maximum

Overdrive

(in)

Fastener

Number

at max.2

Minimum

Overdrive

(in)

Fastener

Number

at min.2

AVG

Overdrive

(in)

Std

Dev

(in)

COV

Initial

Failed

Fastener2

Overdrive

at initial

failure

(in)

1 0.253 35 0.132 9 0.209 0.029 14% 16 0.198

2 0.266 35 0.142 38 0.204 0.030 15% 34 0.244

3 0.271 32 0.154 23 0.208 0.029 14% 31 0.2711Target overdrive = 0.50 x 7/16-inch = 0.219 inches2See Appendix A for fastener numbering and locations on test panel.


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2.2 SHEATHING UPLIFT TESTS

Test Specimens

The sheathing uplift test specimens were framed with five 4-foot 6-inch long, nominal 2x6,Spruce-Pine-Fir (SPF), #1/#2 grade rafters, spaced 24 inches on center. One 4-foot by 8-foot

sheet of 7/16-inch-thick, APA Span Rated 24/16, Exposure 1, OSB was fastened to the rafters

with 8d common pneumatic-driven nails (0.131 x 2.5), with full round heads in accordancewith the variances established in the test matrix (Table 3.1). The rafters and sheathing were

purchased from a local home improvement store and stored inside the NAHB Research Center

laboratory for approximately three weeks prior to the construction of the specimens. Themoisture content was measured at the time of construction and was found to be less than ten

percent in all rafters. The specimens were tested within 48 hours after construction. After all

testing was complete; the framing members were randomly sampled for specific gravity. Theaverage specific gravity was 0.40,which is within four percent of the published specific gravity

value (0.42) for SPF lumber [6].

All nails were installed with a pneumatic gun. For the overdrive tests, a special installationmethod for controlling the penetration depth was used. A weight was positioned on the back end

of the gun to achieve the desired overdrive. Several trial installations were conducted with

varying weights until the target penetration depth was achieved.

Test Apparatus

The uplift testing apparatus was designed to apply an equally distributed uplift pressure on the

under-side of a sheathing panel. A steel frame, anchored to the floor was used to secure the endsof the rafters. An oversized, 8-mil plastic bag was placed in each of the four bays between the

rafters and sheathing was attached to the framing to create four closed chambers containing the

plastic bags. A foam insert was placed inside each bag to pre-form the bags to chambers. The plastic bags were attached to nozzles that protruded through the end of the steel frame. Each

nozzle was connected to a polyvinyl chloride (PVC) pipe, which joined to a common PVC

manifold. The manifold was connected to a variable pressure blower with a water manometer

between the blower and the manifold (see Figures 2.6 and 2.7). The pressure in the system wasmonitored with the water manometer and controlled with manual valves on the blower.


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Figure 2.6 Testing apparatus without sheathing

Figure 2.7 Specimen setup with sheathing attached

PVC

Manifold

Blower

Manometer

Rafters

Sealed Bags

with

Foam Inserts


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Test Procedure

Pressure was applied to the specimen at an increase rate of 20 pounds per square foot per minute

(20 psf/min), which was controlled by manually adjusting an intake valve on the blower while

monitoring the water manometer. Two researchers observed the specimen as the pressure in the

system was increased to record the location and mode of initial failure. The time between initial pressure application and specimen failure was approximately ten minutes. Failure was

determined by a decrease in pressure by at least 50 percent of the observed ultimate pressure, or

by a total loss of fastener connection at one or more locations. Failure modes included nailwithdrawal from the framing, and the nail-head pulling through the sheathing panel. The

maximum pressure was noted at the time of failure. The date, ambient temperature, relative

humidity, and failure mode(s) were recorded for each tested specimen.

2.3 DIAPHRAGM SHEARTESTS

Test Specimens

The diaphragm shear tests were conducted using partial roof diaphragms. These eight-foot by

eight-foot diaphragm sections were tested in a vertical position with the framing membersrunning horizontally. The bottom rafter was bolted to a rigid support and load was applied to the

top rafter in a horizontal direction. The ends of the intermediate-framing members were

restrained with blocking to prevent vertical displacement. The diaphragm sheathing wasconnected to the horizontal-framing members only, and not to the blocking or the vertical end

members which supported the blocking. Steel straps were used to connect the ends of the rafters

to the vertical end members and prevented the rafters from pulling out of the blocking.

The test specimens consisted of five, eight-foot, nominal 2x6, SPF #1/#2 grade, rafters, spaced24 inches on center to create an 8-foot by 8-foot unblocked diaphragm frame. One 4-foot by 8-

foot sheet, and two 4-foot by 4-foot sheets of 7/16-inch-thick, APA Span Rated 24/16, Exposure

1, OSB were fastened to the rafters with 8d common, full round head, pneumatic-driven nails(0.131 x 2.5), in accordance with the variances established in the test matrix (Table 3.1). The

rafters and sheathing were purchased from a local home improvement store and were stored

inside the NAHB Research Center laboratory for approximately three weeks prior to the

construction of the specimens. The moisture content was measured at the time of constructionand was found to be less than ten percent in all rafters. The specimens were tested within 48

hours after construction. After all testing was complete; the framing members were randomly

sampled for specific gravity. The average specific gravity was 0.40,which is within four percentof the published specific gravity value (0.42) for SPF lumber [6].

Test Apparatus

The diaphragms were tested in a shear racking device, which applied lateral load with a servo-

controlled hydraulic ram (see Figure 2.8). The ram had a total travel range of 12 inches, and was

connected to an eight-foot long, 1/4-inch-thick steel plate that was attached throughout to the toprafter with 1/2-inch diameter lag screws spaced 6 inches on center. Two sets of rollers were

located at the top of the diaphragm frame to prevent out of plane movement. Four, 1/2-inch-

diameter bolts, spaced approximately 30 inches on center, were used to attach the bottom rafterto the stationary test platform. A nominal 2x4 wood spacer was placed between the bottom rafter


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and the platform to allow for sheathing rotation. A hold-down bracket was attached to the left

end-member to resist the overturning moment. An electronic load cell with a maximum capacityof 100,000 lb was used to measure the load.

Figure 2.8 Diaphragm specimen in shear racking apparatus

Test Procedure

Load was applied to the specimens by displacing the hydraulic ram at a constant rate of 0.3

inches per minute until failure. Failure was defined by a decrease in load by at least 25 percent of

the observed ultimate load. Data was collected by a computer-based acquisition system [7]. Thetesting date, ambient temperature, relative humidity and failure modes were recorded for each

specimen tested.

3.0 Test Results

3.1 UPLIFT TEST RESULTS

Twenty-three uplift tests were completed. Figure 3.1 shows the locations where the initial

failures occurred. Nineteen of the specimens failed in Region A, one failed in Region B, andthree failed in Region C. Five of the failures which started in Region A initiated at the second

fastener from the end, ten initiated at the third fastener, and four at the fourth fastener. Two

failure modes were observed: nail withdrawal and nail head pull-through. After the first nail

Blocking to restrain rafters

(Sheathing not attached)

Steel Ties

Load Cell

Hydraulic

Ram

Hold-down

Bracket

Unblockedpanel edges


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began to fail, successive nail failures immediately followed, causing a complete specimen failure

within five seconds. Table 3.1 is a summary of the uplift test results.

Figure 3.1 Uplift failure locations (Regions A, B and C)


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TABLE 3.1

UPLIFT TEST RESULTS

Test

Number

Tested

Characteristic

Ultimate

Pressure

(psf)

Average

Pressure

(psf)

Standard

Deviation

(psf)

COVInitial Failure

Location1Failure Mode

1 209 Region A Pull Through

2 237 Region A Withdrawal

3

Baseline

237

228 16 7 %

Region A Pull Through

4 135 Region C Pull Through

5 123 Region C Pull Through

6

Edge Distance #1

(1/8)48

102 47 46 %

Region C Pull Through


8

Edge Distance #2(1/4) 259

248 16 7 %Region A Pull Through



11

Edge Distance #3

(3/4)216

224 7 3 %


12 243 Region B Withdrawal


14

Spacing #1(6.5)

253

245 8 3 %




17

Spacing #2

(7.0 inch)206

211 20 10 %




20

Overdrive #1

(25%)181

182 29 16 %




23

Overdrive #2

(50%)119

108 19 17 %

Region A Pull Through1See Figure 4.1 for region description.

Figures 3.2 and 3.3 show an uplift test in progress. Figure 3.2 shows the OSB deflecting just

prior to failure, and Figure 3.3 shows the specimen immediately after failure.

Figure 3.2 Specimen near ultimate load Figure 3.3 Specimen immediately after failure

Initial failure location

(Region A)


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3.2 CALCULATED UPLIFT CAPACITY

Calculated uplift capacities based on NDS [6] equations were compared to the capacities

observed in the tests. The theoretical ultimate uplift pressure was determined using the calculated

nail withdrawal strength and the assumed tributary loading area of the critical nail. The nail

supporting the largest tributary loading area was assumed as the critical nail. The designwithdrawal strength was calculated using Equation (1) [6], which is a function of the nail

diameter and the specific gravity of the main member. The ultimate withdrawal capacity was

determined with Equation 2, which applies a factor of five to the design strength [8]. Theultimate uplift load per nail was calculated with Equation 3, and the ultimate uplift pressure on

the sheathing was then determined with Equation 4, by dividing the ultimate nail withdrawal

value by the tributary area assigned to the critical nail.

where:

W = design nail withdrawal strength [6]

Wult = ultimate nail withdrawal strength (lb/inch of penetration) [8]G = specific gravity of main member (oven dry) [6]

D = nail diameter (inches)L

p= nail penetration into main member (inches)

Pult = ultimate nail withdrawal force (lb)

Atrib = tributary loading area for nail (ft2)

Pult = ultimate uplift pressure for sheathing connection (psf)

For this test plan, the variables were:

G = 0.40 (average specific gravity of tested lumber)

D = 0.131 inches (8d common nail)

L = 2.06 inches (nail length of 2.5 inches minus 7/16-inch sheathing thickness)Atrib = 0.94 ft

2(tributary area for critical nail. See Appendix C)

DGW 25

1380=

pultult LWP =

trib

ultult

A

P=

131040013805 25

.).( =ultW

inchlbsWult 591.=

062591 .. =ultP

lbsPult 188=

(1)

(3)

(4)

WWult = 5 (2)


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The calculated ultimate pressure underestimated the average tested ultimate pressure (200 psf

calculated, verses 228 psf average tested) by 14 percent.

3.3 SHEARTEST RESULTS

The partial diaphragms were designed to simulate the shear forces that occur in a full diaphragm.However, the partial diaphragms exaggerated weaknesses in edge nailing, and therefore were

considered to provide conservative test results (i.e., ultimate test values were expected to be

lower than ultimate values derived from code allowed design values).

Six diaphragm specimens were tested to failure (see Figure 3.4). Figure 3.5 shows the locationswhere the initial failures occurred. All six specimens initially failed at the bottom of the

diaphragm (regions D and F), where the diaphragm was bolted to the rigid support. Overall, thediaphragms exhibited ductile type behavior, sustaining nearly 80 percent of the ultimate load at

maximum displacement (approximately 8 inches). The initial failure for each of the specimens

was nail tear-through at the panel edges (Figure 3.6 through 3.9). After the initial nail tear-through failure, some specimens exhibited rafter splitting (Figure 3.10). Table 3.2 is a summary

of the shear test results.

Load Direction

Figure 3.4 Diaphragm specimen under load

940

188

.=ult

psfult 200=


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Figure 3.5 Failure locations (Regions D,E,F, and G)

Figure 3.6 Fastener tear-through failure at region D


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Figure 3.7 Fastener tear-through failure at region E

Figure 3.8 Fastener tear-through failure at region F

Figure 3.9 Fastener tear-through failure at region G


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Figure 3.10 Rafter splitting and tear-through

TABLE 3.2

SHEAR TEST RESULTS

Test

Number

Tested

Characteristic

Ultimate

Load (lbs)

Average

Load

(lbs)

Standard

Deviation

(lbs)

Coefficient

of VarianceFailure Mode

1 2,494 Tear-through

2Baseline

2,6562,575 115 4%

Tear-through


4Overdrive #1

2,6022,418 260 11%

Tear-through


6Edge Distance #2

2,6102,550 85 3%

Tear-through

4.0 Evaluation of Results

4.1 UPLIFT TESTS

The ultimate uplift capacities for the various nailing characteristics were compared with thebaseline ultimate capacity. Table 4.1 shows the percent change in capacity from baseline for each

of the nailing characteristics.TABLE 4.1

CHANGE IN UPLIFT CAPACITY FOR TESTED NAILING CHARACTERISTICS

Tested CharacteristicUltimate Uplift

Pressure (psf)COV

Percent Strength Loss (-)

or Gain (+) from Baseline

Baseline 228 7 % --

Edge Distance #1 102 46 % - 55 %

Edge Distance #2 248 7 % + 9 %

Edge Distance #3 224 3 % - 2 %

Spacing #1 245 3 % + 7 %

Spacing #2 211 10 % - 7 %

Overdrive #1 182 16 % - 20 %

Overdrive #2 108 17 % - 53 %


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17

4.2 SHEARTESTS

The ultimate shear capacities for the tested nailing characteristics were compared with the

baseline ultimate capacity. Table 4.2 shows the percent change in capacity from baseline for each

of the tested characteristics.

TABLE 4.2

CHANGE IN SHEAR CAPACITY FOR TESTED NAILING CHARACTERISTICS

Tested Characteristic Ultimate Load (lb) COVPercent Loss (-) or Gain (+)

from Baseline

Baseline 2,575 5 % --

Overdrive #1 2,418 11% - 6 %

Edge Distance #2 2,550 3 % - 1 %

The wind speed that would cause roof sheathing to blow off was assumed to be lower than the

wind speed that would cause a roof diaphragm to fail in shear (i.e., sheathing will typically blow-

off before a roof diaphragm fails in shear). Therefore, the major focus of this research was onuplift testing. The assumption that sheathing would blow-off first was analytically substantiated

by back-calculating the diaphragm shear load that would occur at a wind speed high enough toblow off the sheathing. The average ultimate uplift pressure obtained in the Baseline tests served

as the starting point for this calculation. A velocity pressure was determined using ASCE-7 [9],Components and Cladding (C&C) wind load equations and various building geometry

assumptions (see Appendix B). The calculated velocity pressure was then used with ASCE-7,Main Wind Force Resisting System (MWFRS) equations to determine the diaphragm shear load

at that wind speed. This calculated load represented the shear that would occur in a roof

diaphragm when the wind speed was high enough to blow off the sheathing panels. This wascompared with ultimate diaphragm shear values derived from appropriately adjusted diaphragm

design values. Historically, diaphragm design values have incorporated safety factors in therange of 3.0 to nearly 6.0 [10]. Therefore, upper and lower bound diaphragm capacities werepredicted by applying these factors, along with adjustments for lumber species and loading type,

to the published design values:

Low 230 x 3.0 x 0.9 = 621 plfHigh 230 x 6.0 x 0.9 = 1242 plf

This represents the loading range in which shear failure would be expected to occur. Table 4.2

shows the calculated diaphragm shear based on the velocity pressure associated with each of the

uplift tests. In addition, the expected range for diaphragm shear capacity is included forcomparison. In general, the calculated shear loads based on the ultimate uplift wind speeds are

below, or at the lower end of the ultimate shear range. Therefore, it is reasonable to assume that

roof sheathing will blow-off before the diaphragm will fail in shear. Consequently, for this test

program, only nailing characteristics that had potential to cause large reductions in lateralcapacity were tested in shear. These included panel edge spacing, and fastener overdrive.

Expected shear capacityAdjustment for SPF Lumber (S.G. = 0.42)[6]

Safety factor from APA tests [10]Tabulated allowable diaphragm shear value(IBC-2003)[11]


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18

TABLE 4.2

CALCULATED VELOCITY PRESSURE AND SHEAR LOADS

Tested CharacteristicUltimate Uplift

Pressure (psf)

Calculated Velocity

Pressure (psf)

Calculated Shear

Load (plf)

Published

Diaphragm Shear

Capacity1(plf)

Baseline 228 82 621

Edge Distance #1 102 37 278Edge Distance #2 224 81 611

Edge Distance #3 248 89 677

Spacing #1 245 88 668

Spacing #2 211 76 576

Overdrive #1 182 65 497

Overdrive #2 108 39 295

Range

621 to 1242

1Based on IBC (2003) design value of 230 plf for an unblocked diaphragm with 7/16-inch structural panels and 8d common nails. Base value is adjusted

with 0.90 factor for SPF lumber, and a load factor varying from 3 to 6.

The partial diaphragm specimens performed closely to how full diaphragms would be expected

to perform (i.e., ductile behavior). However, the partial diaphragms showed lower unit strengths.

The capacity changes observed when the nailing variations were introduced were similar to thoseseen in the uplift tests. The changes were within the variability of the baseline tests and did not

show unique responses to the altered nailing. Therefore, uplift was still considered to be thecritical failure mode for any given wind pressure.

4.3 EDGE DISTANCE EFFECTS

Varying the edge distance did not affect the panel uplift connection capacity significantly until

the edge distance was 1/8 inch. The 3/4-inch edge distance (Edge Distance #3) had an averagecapacity reduction of 3 percent, and the 1/4-inch edge distance (Edge #2) had a 9-percent

increase in capacity. However, the initial connection failures for these tests occurred at

intermediate framing members, where the edge spacing had not been altered. Therefore, theintermediate connections governed the uplift capacity and edge spacing from 1/4 inch to 3/4 inch

could be considered acceptable without compromising strength. On the other hand, the 1/8-inch

edge distance tests (Edge # 1) had initial failures on the panel edges. Large reductions in capacitywere observed (over fifty percent) with this edge distance characteristic. Therefore, the 1/8-inch

edge distance was interpreted as unacceptable. Moreover, this nail placement practice damaged

the edge of the panel (see Figure 4.1). It was difficult to drive the nail with 1/8-inch edgedistance without fracturing the edge of the panel.

For shear, nail tear through was consistently the mode of failure. At -inch edge distance, thediaphragm shear tests showed only a 2-percent reduction in capacity. This was within the

variability of the baseline shear tests of 5 percent. The larger edge distance characteristic (3/4inch) was assumed to have negligible affects on the shear capacity due to the increased resistanceto tear-through.


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19

Figure 4.1 Ineffective nailing for Edge Distance #3

4.4 NAIL SPACING EFFECTS

Varying the nail spacing did not significantly affect the uplift capacity of the sheathing

connection. Spacing #1 (nail spacing increased by 1/2 inch) had a seven percent increase instrength, and Spacing #2 (nail spacing increased by 1 inch) had a seven percent decrease in

strength when compared to the baseline tests. Since the capacity changes were within the

variability of the baseline test results, the altered nail spacing was assumed to have no affect on

the connection capacity and a one-inch tolerance for nail spacing could be established as areasonable limit. The total number of nails per rafter would remain the same, but the placement

accuracy could be relaxed to plus-or-minus one inch. Since the same number of nails will remain

in each rafter, shear strength is assumed unchanged. Therefore, no shear tests were conducted forthis nailing characteristic.

4.5 NAIL OVERDRIVE EFFECTS

The specimens with overdriven nails had significant reductions in capacity compared to the

baseline tests. A near linear relationship between percent overdrive and percent capacity loss was

observed (see Figure 4.2). Therefore, it was concluded that nail overdrive should be keptminimal, with a possible tolerance limits set at less than 10 percent. Although, if thicker than

required sheathing were to be used, an overdrive that offsets the increased thickness may be

acceptable.


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20

0%

10%

20%

30%

40%

50%

60%

0% 10% 20% 30% 40% 50%

Overdrive Depth

LossinCapacity

Figure 4.2 Overdrive depth to capacity loss relationship

5.0 Conclusion and Recommendations

The results of this testing plan have shown that moderate variations in certain nailingcharacteristics have only minor affects on ultimate roof sheathing connection capacities.

Tolerance limits for these nailing characteristics can be established with reasonable confidence

using this test data. The implementation of these tolerance limits into installation specificationsor quality-framing programs will enable builders to establish realistic framing goals and give

building officials guidance as to what is acceptable and what is not.

The testing in this plan was limited to one type of sheathing and one nail type, but could provide

insight into expected performance of other nail sizes. Nail diameters that exceed an 8d common

diameter (0.131) are likely to be more sensitive to closer edge spacing and may require tighter

tolerances. Additional testing should be done if these nails are to be included in the tolerancelimits.

Table 5.1 summarizes the recommendations for nailing tolerances for panel edge spacing,spacing between nails, and nail overdrive for 8d common nails or smaller and 7/16-inch thick

OSB sheathing.


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21

Minimum

Maximum

TABLE 5.1

SUGGESTED NAILING TOLERANCES

CURRENT

GUIDELINES

SUGGESTED TOLERANCES

(NAIL DIAMETERD 0.131 AND SHEATHING THICKNESS T7/16)Edge Distance:

3/8-inch from edge of panel to

centerline of fastenerTolerance:

Minimum = 1/4-inch from edge ofpanel to centerline of fastener

Maximum = 3/4-inch from edge of

panel to centerline of fastener andfastener angled such that no portion

is exposed from the underside andthe framing member remains un-

split.

Spacing Between Fasteners:Spacing as specified (e.g., 6

inches on center) Tolerance:

1. Spacing between any two

consecutive fasteners not to exceed

specified spacing plus one inch.

and

2. Total distance between any threeconsecutive fasteners not to exceedminimum spacing multiplied by two,

plus two inches.

and

3. Total number of fasteners in a nail

line shall not be less than calculated

using the minimum required spacing.

Fastener Overdrive:

Head of fastener is to bedriven flush with sheathing.

Tolerance:

Overdrive allowed if sheathing has been sized thicker than required by

design. Depth of penetration must be

kept such that remaining thicknessunder the head of the nail is greater

than or equal to the required

thickness but shall not exceed 1/3the thickness of the panel.


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21

6.0 References

[1] NAHB Research Center, Inc. 1993. Assessment of damage to single-family homes caused

by hurricanes Andrew and Iniki. Prepared for the U.S. Department of Housing and Urban

Development by the NAHB Research Center, Inc., Upper Marlboro, MD.

[2] NAHB Research Center, Inc. 2002. Housing Performance Assessment Report: F-4 La Plata

Tornado of April 28, 2002. NAHB Research Center, Inc., Upper Marlboro, MD.

[3] APA. 1999. Proper Installation of APA Rated Sheathing for Roof Applications. American

Plywood Association.

[4] International Code Council (ICC), International Residential Code 2003, (IRC 2003) forOne- and Two-Family Dwellings. ICC, Falls Church, VA

[5] APA. 1993. APA Report T92-28: Roof Sheathing Fastening Schedules for Wind Uplift.

American Plywood Association, Tacoma, WA.

[6] AF & PA, National Design Specification for Wood Construction 1997 (NDS 97).

American Forrest and Paper Association, Washington, DC

[7] IOtech, Inc. 1996. DaqViewTM

v. 5.0. IOtech, Inc., Cleveland Ohio.

[8] AF & PA, Commentary on the National Design Specification for Wood Construction 1997.

American Forrest and Paper Association, Washington, DC

[9] ASCE. 2000. Minimum Design Loads for Buildings and Other Structures. American

Society of Civil Engineers, Reston, VA.

[10] APA. 1997. APA Report 138: Plywood Diaphragms. American Plywood Association,

Tacoma, WA.

[11] International Code Council (ICC), International Building Code 2003, (IBC 2003). ICC,

Falls Church, VA

[12] NAHB Research Center. 2001. Structural Design Loads for One- and Two-FamilyDwellings. National Association of Home Builders Research Center, Upper Marlboro, MD.


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22


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A-1

Appendix A

Panel Withdrawal Tests - Fastener Numbering

Figure A-1 shows the fastener numbering method used in the withdrawal tests to identify the

location of each nail. Bold vertical lines represent the five rafters, and numbers identify the nails.

1 10 19 28 37

2 11 20 29 38

3 12 21 30 39

4 13 22 31 40

5 14 23 32 41

6 15 24 33 42

7 16 25 34 43

8 17 26 35 44

9 18 27 36 45

Figure A-1

Uplift testing numbering system


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A-2


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B-1

Appendix B

Estimated Wind Speeds and Diaphragm Shears

The following is an example calculation showing the method used to determine an equivalent

wind speed and corresponding diaphragm shear load given an ultimate uplift pressure on a 4-ft. x

8-ft. roof sheathing panel.

Building Assumptions:

(1) Enclosed Building

(2) Number of Stories = 3(3) Length = 50 feet

(4) Width = 25 feet

(5) Roof Pitch = 4:12 (Roof height = 4.2 feet)(6) Exposure = B

(7) Story Height = 10 feet

(9) Uplift Failure Pressure = 228 psf (Average ultimate uplift pressure from testing)(10) Roof Zone = 3

Effective Component and Cladding Area (C & C Area) for individual nails spaced 6 inches on

center with rafters spaced 24 inches on center:

2 ft x 0.5 ft = 1.0 ft2

Roof uplift

Components and Cladding:

External Pressure Coefficient (GCp) = -2.6 (components and cladding zone 3)

Internal Pressure Coefficients (GCpi) = 0.18 (enclosed building)Main Wind Force Resisting SystemLateral Pressure Coefficient Wall (GCp) = 1.1 (Comb. windward and leeward walls)

Lateral Pressure Coefficient Roof (GCp) = 0.5 (Roof slope = 6:12 or less)

Topographic Factor (Kzt) = 1.0

Velocity Pressure

qh = p / (GCp GCpi) = 228 psf / [ (-2.6) (0.18) ] = -82 psf

Velocity wind pressure is the same for the components and cladding and the main wind force-resisting system (a constant velocity pressure exposure coefficient is assumed for both loading

conditions).

Design wind pressure for the main wind force resisting system

Walls: p = (82)( 1.1) = 90 psf

Roof: p = (82)(0.5) = 41 psf


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B-2

Diaphragm unit shear load (occurs at ultimate uplift capacity)

P = [(50)(5)(90)+(50)(4.2)(41)]/(2)/(25) = 622 lb/ft

Diaphragm unit shear capacity range

RLow = (230)(0.9)(3.0) = 621 lb/ft

RHigh = (230)(0.9)(6.0) = 1242 lb/ft

where:

230 lb/ft = allowable tabulated diaphragm shear value (IBC-2003)

0.9 = adjustment factor for SPF with specific gravity of 0.40 (IBC-2003)3.0 = ratio of capacity unit shear to allowable unit shear; this ratio was

adopted as a minimum ratio from test data reported in APA Report 138.

6.0 = ratio of capacity unit shear to allowable unit shear; this ratio wasadopted as the maximum ratio from test data reported in APA Report 138.

This comparison confirms that for the roof configurations investigated, uplift loading will

typically govern the performance of roof sheathing connections (i.e., sheathing panel is expectedto get blown off the rafters before the capacity of the roof diaphragm is exceeded).


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C-1

Appendix C

Tributary Loading Areas for Nail Withdrawal

The total uplift pressure on a roof system is slightly higher than the uplift pressure that initiates

nail withdrawal in the sheathing. This is because the uplift pressure exerted on the roof system is

created by the pressure differential that occurs as air flows over the surface of the roof. The fastermoving air on the outside surface of the roof creates a lower air pressure and therefore an upward

force on the underside of the roof where higher air pressure remains (Figure C-1). The upwardforce is exerted on the entire underside of the roof system, including the underside of the rafters.

A pressure differential does not occur between the rafter and the sheathing until withdrawal of

the nail begins and a gap forms. Therefore, when calculating the tributary loading area for nail

withdrawal (Atrib), the area where the rafter is in contact with the sheathing must be subtracted(Figure C-2).

LowerPressure

HigherPressure

Subtract 1-1/2

Figure C-1 Pressure differential due to wind

Figure C-2 Tributary area adjustment for nail withdrawal


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C-2


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D-1

Appendix D

Metric Conversion Factors

The following list provides the conversion relationshipbetween U.S. customary units and the International System

(SI) units. A complete guide to the SI system and its use

can be found in ASTM E 380, Metric Practice.

To convert from to multiply by

Length

inch (in.) micron () 25,400inch (in.) centimeter 2.54

inch (in.) meter (m) 0.0254foot (ft) meter (m) 0.3048yard (yd) meter (m) 0.9144

mile (mi) kilometer (km) 1.6

Area

square foot (sq ft) square meter (sq m )

0.09290304 square inch (sq in) squarecentimeter (sq cm) 6.452 square inch (sq in.) square meter(sq m ) 0.00064516square yard (sq yd) square meter (sq m )

0.8391274square mile (sq mi) square kilometer (sq km ) 2.6

Volume

cubic inch (cu in.) cubic centimeter (cu cm)

16.387064cubic inch (cu in.) cubic meter (cu m)0.00001639

cubic foot (cu ft) cubic meter (cu m)0.02831685cubic yard (cu yd) cubic meter (cu m)

0.7645549gallon (gal) Can. liquid liter 4.546gallon (gal) Can. liquid cubic meter (cu m) 0.004546gallon (gal) U.S. liquid* liter

3.7854118gallon (gal) U.S. liquid cubic meter (cu m)0.00378541fluid ounce (fl oz) milliliters (ml) 29.57353

fluid ounce (fl oz) cubic meter (cu m)0.00002957

Force

kip (1000 lb) kilogram (kg) 453.6kip (1000 lb) Newton (N)4,448.222

pound (lb) kilogram (kg)0.4535924

pound (lb) Newton (N) 4.448222

Stress or pressure

kip/sq inch (ksi) megapascal (Mpa) 6.894757kip/sq inch (ksi) kilogram/square 70.31

centimeter (kg/sq cm)

pound/sq inch (psi) kilogram/square 0.07031centimeter (kg/sq cm)

pound/sq inch (psi) pascal (Pa) ** 6,894.757

pound/sq inch (psi) megapascal (Mpa) 0.00689476 pound/sq foot (psf) kilogram/square 4.8824

meter (kg/sq m)

pound/sq foot (psf) pascal (Pa) 47.88

To convert from to multiply

by

Mass (weight)

pound (lb) avoirdupois kilogram (kg)

0.4535924ton, 2000 lb kilogram (kg) 907.1848grain kilogram (kg)0.0000648

Mass (weight) per length)

kip per linear foot (klf) kilogram per 0.001488meter (kg/m)

pound per linear foot (plf) kilogram per 1.488

meter (kg/m)

Moment

1 foot-pound (ft-lb) Newton-meter 1.356(N-m)

Mass per volume (density)

pound per cubic foot (pcf) kilogram per 16.01846cubic meter (kg/cu m)

pound per cubic yard kilogram per 0.5933(lb/cu yd) cubic meter (kg/cu m)

Velocity

mile per hour (mph) kilometer per hour 1.60934(km/hr)

mile per hour (mph) kilometer per second 0.0268

(km/sec)

Temperature

degree Fahrenheit (F) degree Celsius (C) tC = (tF-32)/1.8

degree Fahrenheit (F) degree Kelvin (K) tK= (tF+459.7)/1.8

degree Kelvin (F) degree Celsius (C) tC = (tK-32)/1.8

*One U.S. gallon equals 0.8327 Canadian gallon**A pascal equals 1000 Newton per square meter.

The prefixes and symbols below are commonly used toform names and symbols of the decimal multiples andsubmultiples of the SI units.


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Multiplication Factor Prefix Symbol

1,000,000,000 = 109

giga G1,000,000 = 106 mega M1,000 = 103 kilo k0.01 = 10-2 centi c

0.001 = 10-3

milli m0.000001 = 10

-6micro

0.000000001 = 10-9 nano n

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Roof Sheathing Tolerances